Image guidance in radiation therapy has three goals: 1. to decrease the geometric uncertainties of the dose distribution, 2. to improve the sparing of healthy tissue, and 3. to facilitate additional boost treatment to the macroscopic tumour. An ideal imaging system for image guidance would provide real-time imaging at 3-5 frames per second, excellent soft tissue contrast for tumour visualization, and geometric fidelity of 2 mm, all with n additional non-therapeutic radiation dose to the patient. We hypothesize that real-time magnetic resonance image guidance provides the requisite image quality for a wide variety of tumour types and locations in the body. In support of this hypothesis, we have aligned with a multi-institution collaborative program to research, develop and evaluate all aspects of the technology required to bring MR guidance into the radiation therapy world. In this proposal, we bring our experience with electron optics and x-ray tube development to bear on the challenge of researching and building an MR-compatible linear accelerator for use with the 'in-line MR-Linac' system, as well as for use with our new concept of Robotic Linac Adaptation or 'RLA MRI-Linac'. Our hypothesis is that an electron gun designed for optimal operation in an in-line magnetic field can function as effectively and efficiently as current electron gun designs in non- magnetic field conditions. The specific aims of this project are: 1. to develop an optimized electron gun design for use within a parallel fringe field with strengths up to 0.2 T and 2. to build and verify the us of this gun when placed in parallel at up to 0.2 T fringe magnetic field in the In-Line MRI-Linac and/or the RLA MRI- Linac. The first aim will be accomplished using Finite Element modeling to refine our new, MR-compatible electron gun design to include grid control as well as to ensure compatibility between the gun and the accelerating waveguide. The second aim will be accomplished by working with an electron gun manufacturer to build our optimized design, and then by measuring and verifying the performance of the gun in various external fields using a Faraday cup measurement apparatus. The general problem we address is the focusing of radiation beams on tumors as their anatomy and physiology changes during treatment. We believe that successful completion of this MRI-Linac program could have a direct impact on the treatment and lives of cancer patients in the near future.